Most, but not all tours inside of BitGym have sound.
Guided tours have a coach speaking to you as well as background music. If you can’t hear it, you likely have your phone muted or otherwise silenced.
Normal self paced tours usually, but not always, have “Nature Sounds” which can be toggled inside of the workout options panel. If during a workout you tap the screen to bring up this panel, and you see “Nature Sounds” on the left, make sure it’s highlighted on. If it is present and highlighted you either have your volume down or ringer muted. See the help topic on <unmuting your ringer> if you need help.